Many medical devices are now designed to be implantable, including pacemakers, defibrillators, circulatory assist devices, cardiac replacement devices such as artificial hearts, cochler implants, neuromuscular simulators, biosensors, and the like. Since almost all of the active devices (i.e., those that perform work) and many of the passive devices (i.e., those that do not perform work) require a source of power, inductively coupled transcutaneous energy transfer (TET) and information transmission systems for such devices are coming into increasing use.
These systems consist of an external primary coil and an implanted secondary coil, separated by an intervening layer of tissue. This design generally results in a loosely-coupled transformer with no magnetic shielding. Therefore, transformer parameters, such as mutual and self-inductance values, and the effective series resistance of each coil, can be altered by the presence of conductive objects, for example a metal plate, in the vicinity of the coil. Such parameter changes can result in undesired, and in some cases potentially catastrophic, variations in power delivered to the implanted device. Further, an unshielded primary coil generates a magnetic field which is directed in substantially equal parts toward the secondary coil, where it performs useful work, and way from the secondary coil where the magnetic field energy is substantially wasted. If a higher percentage of the magnetic field from the primary coil could be directed to the implanted secondary coil, the energy required to drive the TET device could be reduced. This could result in the device being driveable from a lower energy, and thus a smaller, lighter and less expensive source, or less drainage on an existing source, facilitating longer battery life between replacement or recharging.
A need therefore exists for an improved primary coil construction for a TET device which both reduces sensitivity of the device to conducting objects in the vicinity of the coils and which increases the percentage of magnetic field generated by the primary coil which reaches the secondary coil, thereby significantly enhancing the energy transfer efficiency of the TET device.